Resistive tape

You used the word "precise", your method is not going to give you precise results. There are a number of ways to get pretty accurate results, the best for the dollar is probably something like counting bars on a strip or powering the unit with a stepper motor and tracking the steps. Tom

Can you use a carbon lead from a mechanical pencil?

Potentiometer with a string and a pulley? There are multi-turn potentiometers out there, by the way.

Optics? Gray code, or a simple bar/stripe counter?

Precise ... aah, that is the real question ... how accurate does this have to be?

Why not just rework a computer mouse! Everything you need is there, you could even use a spare mouse to troubleshoot the software/hardware. It would eliminate the need for an A/D converter.

Yukio YANO

If you want precise, code a strip with punched holes with say 4 holes across and use binary encoding. At each step or floor a set of four photo transistor read the binary number encoded on the strip. That way you just use a geared motor or a stepper and have it stop when it finds the number at the strip. JTT.

well I want to be precise. i will need it to stop at every 5mm, and on the way down ned it to stop at the same spot. Maybe use the same thing they use in wheel encoders ?

ken

It is a for a toy so I think he might need to use the newest cutting edge lasers.

Hi, Ken. Easiest way is to use a gearmotor with a cam to trigger a microswitch to stop the motor after every turn. You can use a relay to override the cam to start for the next turn. I can assure you a toymaker would look at this option first -- it's cheapest and most reliable.

If you want something a little more esoteric, you might want to place magnets on each floor, and have a hall-effect sensor in the elevator (or at slightly greater expense, do the reverse). You might also have a separate sensor or switch for the floor (home position).

If those don't do it, your encoder setup might be relatively simple. By reading quadrature, you can tell distance *and* direction. Again, you'll need a home sensor.

Another possibility would be using a long threaded bolt with a riding nut to get the distance. If you use a metric pitch, your 5mm could be just a certain integral number of revolutions. Again, a switch or sensor to determine revolutions, and one for the home position, and you're done. This means gives you a great mechanical advantage, which can be a blessing or a curse.

For any of this, counting revolutions may be a hassle. If so, you might want to use a stepper motor instead of a gearmotor -- it has a set number of steps per revolution, and you can just count those. Steppers are easy to interface to digital logic or microcontrollers

You'll notice I'm kind of edging away from your original suggestion. The reason is your use of the word, "precise". Resistive tape doesn't exist, and even if it did, it would probably wear out right away from the force and friction of the wiper. But it's not very likely you'll be too satisfied with the precision of any resistive solution that's relatively inexpensive. Various solutions might include gearing to a potentiometer, using resistive wire element with a wiper and a current source. No linearity. Using these to accurately gauge distance won't work. Your results might be repeatable, but they definitely won't be accurate.

The expensive means is to use a linear potentiometer, but they're too expensive for toys. Don't go there.

Hope this has been of help. If one of these ideas sounds promising, and you'd like to know more, feel free to post again.

Cheers Chris

Hey Chris, I like this approach. He could stop the motor on every tooth of a gear but override the action with relay shunt contacts. On the desired floor there could be a microswitch to drop the relay and let the gear stop the motor. I like it.

Tom

I don't see anything popping up like that at Digikey. However, it looks like resistance wire is pretty common. Here's a couple of examples:

Resistance Wire 1.30 ohms/foot 22 ga KAN AF Resistance Wire 5.23 ohms/foot 28 ga KAN AF

Thanks for your ideas. So far I think the best way is using a servo motor with roughly 3% angle shift. I do not want this to be too complicated and costly. I do not know how reliable are servo motors, nerver worked with them. Just a it worried that after many trips up and down it might skip a gear knotch.

ken

On one of those home improvement shows on TeeVee the other day, they had made a dumbwaiter from a garage door opener.

Good Luck! Rich